Laser marking of phosphorescent plastic articles

ABSTRACT

The invention provides molded, extruded or formed phosphorescent plastic articles having laser marked indicia and methods for preparing the articles. The indicia may be seen as a contrast against a phosphorescent background or, alternatively, the indicia may themselves be phosphorescent. An article of the invention has a composition comprising about 50% to about 99% by weight of a thermoplastic or thermosetting resin, about 1% to about 50% by weight of a phosphorescent phosphor pigment and about 0.001% to about 1% by weight of a laser energy absorbing additive. Resin compositions which themselves are laser energy absorbing do not require the additive; however, the addition of small quantities of the laser energy additive enhances the quality and contrast of the laser mark. The composition may further comprise small quantities of one or more of a whitening pigment, a lubricating agent, or an inert filler without interfering with the phosphorescence of the article and/or the quality and contrast of the laser mark.

This application is a continuation of U.S. patent application Ser. No.08/991,705, filed Dec. 16, 1997, now U.S. Pat. No. 5,976,411.

BACKGROUND OF THE INVENTION

Laser marking is a well known and important means for quickly andcleanly inscribing plastic surfaces with permanent informational indiciamarks, such as date codes, batch codes, bar codes or part numbers,functional marks, such as computer keyboard and electronic keypadcharacters, and decorative marks, such as company logos. However, suchlaser marks are visible and readable only in lighted environments. Theinability to read informational indicia on these and other items, suchas clocks, emergency exit signs, safety information signboards, interiorautomobile control buttons, and the like, in a dark or dimly litenvironment is a common problem in modern society. As used in thecontext of the invention, the term “indicia” refers to any laser markwhatsoever including, but not limited to, alphabetical characters,numbers, drawings, patterns, and the like.

It has been the practice to enable visibility of indicia in the dark byproviding electrically powered illumination systems. Some illuminationdevices are self- contained, having a battery and light which clips ontothe article to provide external illumination. Other systems rely on alight source within the article, the light being projected through thewall of the article to provide “back light” illumination of the indiciathereon or of the area immediately surrounding the indicia to providecontrast. Other devices utilize an optical conductor to transmitillumination from a light source to, for example, the keys of a key pad.Such illumination systems, however, whether battery powered orelectrically wired, add bulkiness, weight, expense and maintenance coststo the item. Moreover, in cases of power outages, electrically wiredsystems are useless and many safety items, such as emergency exit signs,require back-up battery systems.

The use of phosphorescent materials in the production of luminescentarticles is very well known, having been used for such diverse“glow-in-the-dark” items as golf balls, rubber shoe soles, manyvarieties of toys, safety helmets, safety tape for bicycle visibility,and the like. Phosphorescent phosphors are substances that emit lightafter having absorbed ultraviolet radiation or the like, and theafterglow of the light that can be visually observed continues for aconsiderable time, ranging from several tens of minutes to several hoursafter the source of the stimulus is cut off. Phosphorescent materialshave been reported for use as night-time illumination for vehicle parts,such as wheel valve stem caps, gear shift knobs, or for phosphorescenttabs for illuminating keyholes. They have also been used for night-timeillumination of plastic electronic key pads that contain informationalindicia attached to the key pads by an adhesive.

Although fluorescent pigments and dyes have been employed in plasticmaterials suitable for laser marking, it has not previously beenconsidered possible to laser mark indicia on phosphorescent plasticarticles. The most common mechanism of laser marking of plasticmaterials depends on the rapid production of heat in the irradiatedportion of the plastic due to the absorption of the laser energy. Manyplastics, such as polyethylene, polypropylene and polystyrene, aretransparent to laser energy at certain wavelengths, and many otherplastics do not absorb laser energy well. In order to laser markplastics such as these, it is necessary to incorporate laser energyabsorbing additives, such as clay, talc, titanium dioxide, carbon black,barium sulfate, and the like, into the polymeric composition. However,it was believed that incorporation of such laser energy absorbingadditives into phosphorescent plastic compositions would mask thephosphorescence or, at the least, significantly interfere with theintensity of the phosphorescence and the duration of the afterglow,requiring uneconomically large quantities of these expensivephosphorescent phosphors to overcome the masking effect of the fillers.

Moreover, until recently, the commonly available phosphorescentphosphors, such as sulfide phosphorescent phosphors, were capable ofemitting a visible afterglow that lasted only from about 30 minutes toabout 2 hours and their use required repeated exposure to a lightstimulus in order for the afterglow to be sustained for practical timeperiod. Such sulfide phosphorescent phosphors include CaS:Bi (whichemits violet blue light), CaStS:Bi (which emits blue light), ZnS:Cu(which emits green light) and ZnCdS:Cu (which emits yellow or orangelight). Because of the short afterglow duration of these phosphors,there were concerns about diminishing the luminescence and afterglowtime by adding laser energy absorbing fillers to polymer compositionscontaining these phosphorescent phosphors.

Recently, however, a new group of phosphorescent phosphors that bothabsorb and emit light in the visible spectrum has become available (seeU.S. Pat. No. 5,424,006). These phosphors comprise a matrix expressed byMAl₂O₄ (where M is a metal such as calcium, strontium, or barium) dopedwith at least one other element, such as europium, ytterbium,dysprosium, thulium, erbium, or the like, to form a phosphorescentphosphor (e.g., SrAl₂O₄:Eu). These phosphors have a crystallinestructure and they are highly chemically stable metal oxides. The newphosphors have an afterglow that is three to more than twelve timesbrighter than that of the comparable sulfide based phosphorescentphosphors at 10 minutes after stimulation, and 17 to more than 37 timesbrighter at 100 minutes after stimulation. Moreover, the afterglow ofthese phosphors is still visibly apparent 15 to 24 hours afterstimulation. The suggested uses for these phosphorescent phosphorsinclude phosphorescent articles such as signs, luminous inks, toys,directional markers, and backlighting for liquid-crystal items such asclocks, electrical appliance switches, key hole indicators, and thelike, but there is no suggestion that plastic articles containing theseor similar phosphorescent phosphors may be suitable for laser marking.

Another group of phosphorescent phosphors containing neodymium andytterbium ions has also been recently described. These phosphors absorblight in the visible spectrum and emit light in the infrared spectrum(see U.S. Pat. No. 5,220,166) and reportedly can be incorporated intoplastic materials during manufacture, into paints used as coatings, orinto ink compositions for printing. The infrared light emitted by thephosphor is detected by a photodetector device, such as a bar codereader, credit card/bank card reader, and the like. However, there is nosuggestion that indicia can be laser marked on such phosphorescentplastics.

SUMMARY OF THE INVENTION

It has been unexpectedly discovered that phosphorescent plastic articlescomprising polymer compositions that also contain laser energy absorbingadditives, with or without the addition of inert fillers, pigmentsand/or other additives, can be laser marked and, contrary toconventional expectations, the addition of these laser energy absorbingadditives, fillers and/or pigments, and the like, in the concentrationsaccording to the invention, does not interfere with the resultingphosphorescence of the article. Molded, extruded or formed plasticarticles containing laser energy absorbing additives and phosphorescentphosphor pigments that emit light in the visible wavelength range can belaser marked to provide permanent informational and other indicia thatare visible with high contrast in light environments and are alsovisible in the dark with high contrast against a phosphorescent plasticbackground. Any thermoplastic or thermosetting resin that isconventionally used in producing plastic articles suitable for lasermarking may be used in the composition of the invention to produce alaser-markable phosphorescent plastic article.

In particular, it has been discovered that laser energy absorbingadditives, such as mica-based additives, carbon black, titanium dioxide,barium sulfate, talc, china clay, and mixtures of these, can be added tothe phosphorescent phosphor pigment-containing polymer compositions in aconcentration of about 0.001% to about 1% by weight, depending on theadditive selected, to provide laser-markable phosphorescent plasticarticles without interfering with the phosphorescence of the article.All weight percentages are herein expressed as the weight percentage ofthe additive in the final plastic part.

It has also been discovered that phosphorescent plastic articlescomprising polymers that are known to readily absorb laser energywithout the addition of a laser energy absorbing additive, such aspolyvinyl chloride, polyethylene terephthalate, acrylonitrile butadienestyrene (ABS), and the like, are laser markable when they contain about1% to about 50% by weight of a phosphorescent phosphor pigment.

It has further been discovered that the polymer compositions mayoptionally further contain about 0.001% to about 2% by weight of awhitening pigment, such as titanium dioxide, zinc sulfide, and the like,about 0.001% to about 20% by weight of an inert filler, such as calciumcarbonate, talc, china clay, and the like, and other additives, such asabout 0.1% to about 10% of a lubricating agent, without interfering withthe phosphorescence of the resulting plastic articles. Theconcentrations of these pigments, fillers, and the like, are far lessthan the concentrations that are conventionally used to make plasticarticles that are not phosphorescent. Contrary to conventionalexpectations, it has been further discovered that the addition ofwhitening pigments to thermoplastic and thermosetting compositionscontaining a phosphorescent phosphor pigment does not substantiallyreduce, and in some cases actually enhances, the phosphorescence of theplastic articles obtained.

It has also been discovered that phosphorescent indicia are producedwhen phosphorescent plastic articles further contain a “masking”pigment, such as carbon black or a color pigment, that decomposes,vaporizes or becomes colorless when exposed to laser energy to exposethe underlying phosphorescent, thus producing a phosphorescent indiciaagainst a dark or colored background. When masking pigments are used,the ratio of the phosphorescent phosphor pigment to the masking pigmentis about 150:1 to about 1:1. A phosphorescent indicia may also beproduced when a phosphorescent phosphor pigment-containing plasticarticle is coated with an opaque color pigment or dye (e. with a paintor ink) that decomposes, vaporizes or becomes colorless when exposed tolaser energy to reveal the underlying phosphorescence.

When crystalline phosphorescent phosphors, such as those described inU.S. Pat. No. 5,424,006, are employed in polymer compositions, the lasermarked plastic articles obtained have vastly improved luminescence inthe dark and prolonged afterglow durations. When visible lightabsorbing, infrared light emitting phosphorescent phosphors, such asthose described in U.S. Pat. No. 5,220,166, are employed in polymercompositions, the obtained plastic articles may be laser marked and theinfrared emission read by a photodetector designed to detect light withwavelengths between 840 nm and 1100 nm. As described above, the indiciamay be read as a contrast against the phosphorescent background, orindicia may be themselves phosphorescent against a masked background.Thus, the plastic articles may be laser marked with bar coding or otherindicia that are read by photodetector devices.

It has also been discovered that both light and dark laser marks ofvarying shades and intensities can be obtained on phosphorescent plasticarticles by varying and controlling laser parameters, such as amperage,pulse frequency, pulse duration, energy density, peak power, pulseenergy, and the like. Thus, plastic articles with laser marked indiciahaving two or more preselected light or dark contrasting shades may beobtained to produce multi-colored designs visible in light environments.

Because phosphorescent phosphor pigments are produced by processes thatcan achieve temperatures of 700° C. to 1300° C. or more, plasticarticles containing these pigments can be manufactured by any extrusion,molding or forming process without decomposition of the phosphor.Moreover, any type of laser including, but not limited to, Nd:YAG(Neodymium doped Yttrium Aluminum Garnet), CO₂ and excimer lasers may beused to mark the resulting phosphorescent phosphor pigment-containingplastic article.

The laser marked phosphorescent articles of the invention may be usedfor any application in which it is necessary for such indicia to bevisible in dark or dimly lit environments, including, but not limitedto, keypad characters on telephones, cellular telephones, pagers, andthe like, computer keyboard characters, clocks including liquid-crystalclocks, emergency exit signs and other safety signage in buildings,hotels, aircraft, ships, and the like, safety directional signboardssuch as for fire extinguisher locations, signs inscribed with safetyinformation that is necessary even during power outages, for instrumentpanels and controls of vehicles including motorcycles, and furtherincluding interior vehicle buttons having informational indicia, such aspower window, door lock and mirror controls, heating and airconditioning controls, radio controls, windshield wiper/washer controls,and the like.

The laser marked phosphorescent plastics of the invention eliminate thebulkiness, weight, expense and maintenance costs of current battery orelectrically powered lighting systems, and eliminate illuminationproblems associated with power outages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are photographs illustrating both light and dark lasermarking of various shades and intensities produced on phosphorescentpolycarbonate (PC) articles without (1A) and with (1B) the addition of alaser energy absorbing additive LS820, made with the formulationpresented in Table 1.

FIGS. 2A and 2B are photographs illustrating both light and dark lasermarking of various shades and intensities produced on phosphorescenthigh impact polystyrene (HIPS) articles without (2A) and with (2B) theaddition of a laser energy absorbing additive LS820, made with theformulation presented in Table 1.

FIG. 3 is a photograph illustrating both light and dark laser marking ofvarious shades and intensities produced on a phosphorescent high densitypolyethylene (HDPE) article with the addition of a laser energyabsorbing additive LS820, according to the formulation presented inTable 1.

FIG. 4 is a photograph illustrating both light and dark laser markingsof various shades and intensities produced on a phosphorescent HIPSarticle containing 0.03% zinc sulfide as a whitening agent.

FIG. 5 is a phpotograph illustrating both light and dark laser markingsof various shades and intensities produced on a phosphorescent HIPSarticle containing 0.01% titanium dioxide as a whitening agent.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “comprising” means that the listed components,ingredients or steps do not exclude other components, ingredients orsteps known to one skilled in the technology that may be added forconvenience or designed for certain end use applications, as long asthese do not interfere with the absorption and emission of light fromthe phosphorescent phosphor pigment and the laser marking of theresulting plastic article. Accordingly, the term “comprising”encompasses the more restrictive terms “consisting essentially of” and“consisting of”.

In one embodiment, the invention provides a molded, extruded or formedphosphorescent plastic article having a laser marked indicia thereon andhaving a composition that comprises (i) about 50% to about 99% by weightof a thermoplastic or thermosetting resin, (ii) about 1% to about 50% byweight of a phosphorescent phosphor pigment, and (iii) about 0.001% toabout 1% by weight of a laser energy absorbing additive. The compositionmay optionally further contain zero to about 10% by weight of alubricating agent, zero to about 2% by weight of a whitening pigment,and zero to about 20% by weight of an inert filler.

The thermoplastic or thermosetting resin may be any resin usable toproduce plastic articles suitable for laser marking, includingthermoplastic polyolefin resins, such as polyethylene, high densitypolyethylene and polypropylene that are well known to be transparent tolaser energy from a Nd:YAG laser and require a laser energy absorbingadditive. The usable resins further include polystyrene which requires alaser energy absorbing additive to produce a plastic article suitablefor marking with a CO₂ laser. The addition of the laser energy absorbingadditive in the described concentration does not diminish or interferewith the brightness of the luminescence or the duration of the afterglowin the resulting laser marked phosphorescent plastic article.

Polymer resins suitable for use in this embodiment of the inventioninclude thermoplastic, thermosetting, addition and condensationpolymers. Illustrative examples include, but are not limited to,polyesters, polystyrene, high impact polystyrene, styrene-butadienecopolymers, impact modified styrene-butadiene copolymer, poly-α-methylstyrene, styrene acrylonitrile copolymers, acrylonitrile butadienecopolymers, polyisobutylene, polyvinyl chloride, polyvinylidenechloride, polyvinyl acetals, polyacrylonitrile, polyacrylates,polymethacrylates, polymethylmethacrylates, polybutadiene, ethylenevinyl acetate, polyamides, polyimides, polyoxymethylene, polysulfones,polyphenylene sulfide, polyvinyl esters, melamines, vinyl esters,epoxies, polycarbonates, polyurethanes, polyether sulfones, polyacetals,phenolics, polyester carbonate, polyethers, polyethylene terephthalate,polybutylene terephthalate, polyarylates, polyarylene sulfides,polyether ketones, polyethylene, high density polyethylene,polypropylene, and copolymers, grafts, blends, and mixtures thereof. Thecopolymers and blends of these polymers are well known to those skilledin the relevant technology, as are polymers which may serve as abackbone or a grafted polymer in graft polymers.

It is well known that there are polymers that readily absorb laserenergy from lasers, such as Nd:YAG, CO₂ or excimer lasers, without theaddition of laser energy absorbing additives. Illustrative examples ofsuch laser energy absorbing polymers are polyvinyl chloride,polyethylene terephthalate, ABS, and the like. Thus, in anotherembodiment, the invention provides a molded, extruded or formedphosphorescent plastic article having a laser marked indicia thereon andhaving a composition that comprises (i) about 50% to about 99% by weightof a thermoplastic or a thermosetting resin that absorbs laser energy,and (ii) about 1% to about 50% by weight of a phosphorescent phosphorpigment. This composition may optionally further contain zero to about10% by weight of a lubricating agent, zero to about 2% by weight of awhitening pigment, and zero to about 20% by weight of an inert filler,as well as other additives described below that do not interfere withthe absorption and emission of light from the phosphorescent phosphorpigment and the laser marking of the resulting plastic article. Usefulpolymers that readily absorb laser energy without the addition of alaser energy absorbing additive are known to those skilled in therelevant technology and are suitable for use in this embodiment of theinvention. Such polymers include, but are not limited to, addition andcondensation polymers selected from polyesters, polystyrene, high impactpolystyrene, styrene-butadiene copolymers, impact modifiedstyrene-butadiene copolymer, poly-α-methyl styrene, styreneacrylonitrile copolymers, acrylonitrile butadiene copolymers,polyisobutylene, polyvinyl chloride, polyvinylidene chloride, polyvinylacetals, polyacrylonitrile, polyacrylates, polymethacrylates,polymethylmethacrylates, polybutadiene, ethylene vinyl acetate,polyamides, polyimides, polyoxymethylene, polysulfones, polyphenylenesulfide, polyvinyl esters, melamines, vinyl esters, epoxies,polycarbonates, polyurethanes, polyether sulfones, polyacetals,phenolics, polyester carbonate, polyethers, polyethylene terephthalate,polybutylene terephthalate, polyarylates, polyarylene sulfides,polyether ketones, and, copolymers, grafts, blends and mixtures thereof.

Any type of laser that may be used in marking plastic articles may beused to mark the phosphorescent plastic articles including, but notlimited to, Nd:YAG lasers having principal wavelength of 1,064 nm or 532nm (frequency doubled), solid state pulsed lasers, pulsed metal vaporlasers, excimer lasers, TEA-CO₂ lasers, continuous wave (CW) CO₂ lasers,ruby lasers, diode lasers, and the like, with Nd:YAG, TEA-CO₂ andexcimer lasers being preferred.

Phosphorescent phosphors suitable for use in the invention include anyphosphorescent phosphor pigment that is insoluble in the resin,including conventional sulfide phosphorescent phosphors. The manufactureof phosphorescent phosphor pigments is well known and involves heatingthe components to temperatures of about 700° C. to 1300° C. or higher,cooling the obtained compound and pulverizing and/or sieving (e. to 100mesh) the compound to obtain a powder having a given particle size. Thephosphorescent phosphor pigments are usually colored powders, such asyellowish-green, greenish-blue, orangish-yellow powders, whichthemselves may impart a degree of color to translucent, transparent oropaque plastic articles.

For applications requiring brighter luminescence and longer afterglowdurations than provided by conventional sulfide phosphors,phosphorescent phosphor pigments, such as those described in U.S. Pat.No. 5,424,006, the disclosure of which is hereby incorporated byreference, are preferred. These described phosphorescent phosphorpigments, which absorb visible or ultraviolet light and emit light inthe visible spectrum, are manufactured by Nemoto & Company, Tokyo andare obtainable from United Mineral & Chemical Corporation, Lyndhurst,N.J. under the brand name LUMINOVA®. These phosphors are chemicallystable metal oxides having a crystalline (spinel, monoclinic) structureand particle sizes ranging from about 7 μm to greater than 250 μm. Theycontain elements that include strontium (Sr), aluminum (Al), calcium(Ca), barium (Ba), magnesium (Mg), europium (Eu), lanthanum (La), cerium(Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd),dysprosium (Dy), holmium (Ho), erbium (Er), terbium (Tb), thulium (Tm),ytterbium (Yb), lutetium (Lu), tin (Sn) and bismuth (Bi). The chemicalcompositions of exemplary phosphors suitable for use in the inventioninclude SrAl₂O₄:Eu or SrAl₂O₄:Eu,Dy or SrAl₂O₄:Eu,Nd that emit greenlight, and CaSrS:Bi, CaSrS:Bi,Nd, CaAl₂O₄:Eu, plus Nd, Sm, Dy, Tm, Ce,Pr, Gd, Tb, Ho, Er, Yb, Lu, Mn, Sn, or Bi, that emit blue light, orBaAl₂O₄:Eu,Nd, BaAl₂O₄Eu,Sm, and the like, that emit green light.

LUMINOVA® Green phosphorescent phosphor pigments demonstrate anafterglow duration, after excitation with a standard light source at 400lux for 20 minutes at room temperature, of about 1500 minutes to 2500minutes (25 to 42 hours) or, after excitation with a standard lightsource at 200 lux for 4 minutes, of about 700 minutes to 1000 minutes(12 to 17 hours). For example, LUMINOVA® Green (G-300M) comprisingStrontium Aluminate Oxide:Dysprosium, Europium has a specific gravity ofabout 3.6, a particle size of 20±5 μm to greater than 100 μm and absorbslight in the ultraviolet spectrum (about 320 to about 382 nm). Thispigment emits green light at a peak wave length of about 521 nm with anafterglow duration of greater than 2000 minutes (33 hours) afterexcitation with a standard light source at 400 lux for 20 minutes atroom temperature.

Other suitable phosphorescent phosphor pigments for use in the inventionare manufactured by Chemitech, Inc., Tokyo and available from F.W. BassInternational, Inc., Moon Township, Pa. under the brand name PICARIKO®.For example, PICARIKO® CP-05 is a phosphorescent phosphor pigmentmanufactured from a mixture of aluminum oxide, strontium oxide, calciumoxide, europium oxide, and boron oxide. This phosphor pigment has acrystalline (triclinic) structure and has a melting point of greaterthan 1500° C., a particle size that passes through 200 mesh and aspecific gravity of about 3.6. It is a yellowish green powder thatabsorbs light in the ultraviolet spectrum (200-450 nm) and emits a greenlight having an emission peak of 520 nm with an afterglow duration ofgreater than 20 hours.

Other phosphorescent phosphor pigments available as LUMINOVA® orPICARIKO® emit visible blue, orange, yellowish green light, and thelike.

Another phosphorescent phosphor pigment suitable for use in theinvention is described in U.S. Pat. No. 5,220,166, the disclosure ofwhich is hereby incorporated by reference. This pigment is a phosphate,molybdate or aluminate compound activated by neodymium and ytterbiumions. Examples of these phosphorescent phosphor pigments includeLi(Nd,Yb)P₄O₁₂; Ca₁₀(PO₄)6F₂:Nd,Yb; Ca₈La₂(PO)₄6O₂:Nd,Yb; and the like.These phosphors are excited by visible light, having peaks near 525 nm,585 nm, 630 nm, 685 nm, 745 nm, and near 800 nm and emit light in theinfrared range, with a peak near 980 nm. The infrared emitted light maybe read, for example, by a photodetector designed to detect light withwave lengths between 840 nm and 1100 nm. Thus, phosphorescent plasticarticles having polymer compositions according to the invention thatcomprise these phosphorescent phosphor pigments are suitable for laserinscription of indicia which can be read directly by photodetectordevices, such as bar code readers, and the like. The indicia may be readby contrast against a phosphorescent plastic background or, if theindicia themselves are phosphorescent, they may be read directly.

The selected concentration of the phosphorescent phosphor pigment in thepolymeric composition depends upon the degree of luminescence andduration of afterglow desired. In general, the higher the concentrationof the phosphorescent phosphor pigment, the brighter is the luminescenceand the longer is the afterglow duration. It has been found that aslittle as 1% by weight of a phosphorescent phosphor pigment such asLUMINOVA® or PICARIKO® provides visible luminescence, whereas 30% ormore of a conventional sulfide-based phosphor may be required to producethe same amount of luminescence. Because phosphorescent phosphorpigments such as LUMINOVA® and PICARIKO® are relatively expensive, abalance between the desired luminescence and afterglow duration versusthe expense of the article may dictate a smaller concentration than 50%by weight, such as about 5% to about 20% by weight, preferably about 5%to about 10% by weight, and more preferably about 5% by weight.

It has been discovered that laser energy absorbing fillers, such asmica, carbon black, titanium dioxide, china clay, barium sulfate, talc,and the like, whitening pigments, such as titanium dioxide, zincsulfide, zinc phosphate, lithopone, zinc oxide, barium sulfate, and thelike, inert fillers, such as barium sulfate, calcium carbonate, talc,china clay, and the like, and mixtures of any of the foregoing, may beadded to the polymer composition in concentrations of about 0.001% toabout 50% by weight, depending on the type of filler, withoutsubstantially interfering with the luminescence of the phosphor.Surprisingly, a whitening pigment, such as titanium dioxide in smallamounts (about 0.001% to about 0.1%), has been found to be lightreflecting and, rather than interfering with the luminescence, actuallyenhances it.

Any laser energy absorbing additive known to those skilled in the art oflaser marking may be used in the polymer compositions of the inventionin an appropriate concentration. Preferably, the laser energy absorbingadditive is selected from the group consisting essentially of about0.001% to about 1% by weight of a mica-based laser energy absorbingadditive, about 0.001% to about 0.1% by weight carbon black, about0.001% to about 0.1% by weight titanium dioxide, about 0.01% to about 1%by weight barium sulfate, about 5% to about 20% by weight talc, about 5%to about 20% china clay, and mixtures thereof.

More preferably the laser energy absorbing additive is selected from thegroup consisting essentially of about 0.15% to about 0.5% by weight,especially 0.5% by weight, of the mica-based laser energy absorbingadditive, about 0.001% to about 0.03% by weight carbon black, about0.005% to about 0.05% by weight, especially 0.025% by weight titaniumdioxide, about 0.05% to about 0.5% by weight, especially 0.25% by weightbarium sulfate, about 5% to about 15% by weight talc, about 5% to about15% china clay, and mixtures thereof.

When mica is used as a laser energy absorbing filler, it is preferablyin flake, platelet or pearl form, preferably having a particle size ofless than 15 μm for use with Nd:YAG lasers. Suitable mica-basedadditives, available from EM Industries, Hawthorne, N.Y., for use withNd:YAG lasers include IRIODINO®/LAZER FLAIRS® LS 820 and LS 825.Mica-based additives suitable for use with TEA-CO₂ lasers includeIRIODINE®/LAZER FLAIR® LS 800, LS 805 and LS 810, having a particle sizeranging from less than 15 μm to 110 μm.

The polymer compositions may further comprise about 0.001% to about 2%of a whitening pigment, such as those described above. Preferably, thewhitening pigment is selected from the group consisting essentially ofabout 0.001% to about 0.1% by weight titanium dioxide, about 0.005% toabout 0.03% by weight zinc sulfide, about 0.005% to about 0.03% byweight zinc phosphate, about 0.005% to about 0.03% by weight lithopone,about 0.005% to about 0.03% by weight zinc oxide, about 0.005% to about0.03% by weight barium sulfate, and mixtures thereof.

The polymer composition may further comprise about 0.001% to about 20%by weight of an inert filler, as known to those skilled in the art ofpolymer compounding. Preferably, the inert filler is selected from thegroup consisting of calcium carbonate, talc, china clay, and mixturesthereof.

In order to aid in dispersing the phosphorescent phosphor pigment andother additives in the polymer composition during the compoundingprocess, the composition may further comprise about 0.1% to about 10% ofa lubricating agent. Suitable dispersing/lubricating agents known tothose skilled in the art comprise low molecular weight substantiallytransparent polymeric materials, such as silicone waxes, fatty acids,metallic salts, ionomer waxes, amide waxes, hydroxy stearates, olefinicwaxes, metallic stearates, complex esters, polyethylene waxes,polypropylene waxes, and mixtures thereof. An exemplary suitablelubricating agent is ADVAWAX® 280, obtainable from Morton Thiokol, Inc.,Cincinnati, Ohio, with the chemical name N,N′ ethylene bis(stearamide).

In addition to the resins, laser energy absorbing additives, whiteningpigments, inert fillers and lubricating additives described above, thepolymer composition may comprise other additives provided the additivesdo not interfere with the absorption and emission of light from thephosphorescent phosphor pigment and the laser marking of the resultingplastic article. Such other additives are known to those skilled in theart of polymer compounding and include, but are not limited to,reinforcing fillers, flame retardants, antioxidants, impact modifiers,plasticizers, and the like.

Examples of polymer compositions suitable for producing laser markedmolded, extruded or formed phosphorescent plastic articles areillustrated in Table 1. The phosphorescence of the article, the qualityof the laser mark and the contrast of the laser mark against thebackground were visually graded on a scale of 1 to 5, with 5 being thehighest and 1 the lowest. The laser marks were made on molded articleswith a Nd:YAG laser having a wavelength of 1064 nm, with operatingparameters of 11 amps, 5 KHz frequency, and a velocity of 20 feet perminute. As illustrated in the Table, K-resin, high impact polystyrene(HIPS), general purpose polystyrene (GPS), polycarbonate, styreneacrylonitrile copolymer, polyethylene terephthalate and ABS are lasermarkable with a good quality, high contrast mark with or without theaddition of a laser energy absorbing additive such as a mica-basedadditive LS820. However, a laser energy transparent polymer, such ashigh density polyethylene (HDPE) requires the addition of a laser energyabsorbing additive to produce a high quality, good contrast laser mark.

As further illustrated in Table 1, the addition of the laser energyabsorbing additive does not interfere with the glow effect(phosphorescence) of the phosphorescent plastic articles. Moreover, whena laser absorbing polymer such as polycarbonate is employed, arelatively low amount of a laser absorbing additive noticeably andsignificantly enhances the quality of the laser mark. This result isunexpected. One skilled in the art would not usually add a laser energyabsorbing additive to a laser absorbing polymer such as PC in order toachieve a good quality mark. Because of this surprising discovery thatthe mark quality is enhanced by the addition of a minor amount of theadditive, the total laser energy required to achieve a mark of excellentcontrast may now be reduced. Such a lower laser energy level will resultin a faster processing speed, a longer lifetime for laser equipment and,importantly, less damage to the plastic surface so that the markingquality is drastically improved.

The phosphorescent plastic articles suitable for laser marking includeany plastic articles that are molded, extruded or formed by any knownconventional method. In one embodiment of the invention, a method forproducing a laser marked indicia on a molded, extruded or formedphosphorescent plastic article, comprises the steps of compounding apolymeric composition that comprises (i) about 50% to about 99% byweight of a thermoplastic or a thermosetting resin, (ii) about 1% toabout 50% by weight of a phosphorescent phosphor pigment, and (iii)about 0.001% to about 1% by weight of a laser energy absorbing additive;molding, extruding or forming the compounded composition to form aphosphorescent plastic article; and exposing a portion of the plasticarticle to laser energy from a laser to inscribe an indicia thereon.

In another embodiment of the invention, when the resin is itself laserenergy absorbing, the method for producing a laser marked indicia on amolded, extruded or formed phosphorescent plastic article comprises thesteps of compounding a polymeric composition that comprises (i) about50% to about 99% by weight of a thermoplastic or a thermosetting resinthat absorbs laser energy, and (ii) about 1% to about 50% by weight of aphosphorescent phosphor pigment; molding, extruding or forming thecompounded composition to form a phosphorescent plastic article; andexposing a portion of the plastic article to laser energy from a laserto inscribe an indicia thereon. In each of the above embodiments, thecomposition preferably further comprises about 0.1% to about 10% of alubricating agent.

In the embodiments of the invention, phosphorescent plastic articlescontaining phosphorescent phosphor pigments that emit light in thevisible wavelength range can be laser marked to provide permanentindicia that are visible with high contrast in light environments andare also visible in the dark with high contrast against a phosphorescentplastic background. Alternatively, the laser marked indicia themselvesare phosphorescent against a dark or colored plastic background. In thisembodiment of the invention, the plastic article containing thephosphorescent phosphor pigment in a concentration described above,further comprises a color pigment or carbon black, that decomposes,vaporizes, or becomes colorless when exposed to laser energy. The colorpigment or carbon black is present in a concentration sufficient to maskthe phosphorescence of the background of the plastic article in thedark. The ratio of the phosphorescent phosphor pigment to the colorpigment or carbon black is preferably about 150:1 to about 1:1. When theindicia is laser marked on the masked background of the plastic article,the underlying phosphorescence is exposed, resulting in a phosphorescentindicia. The indicia may or may not be visible in a light environment.However, it is preferable that the laser marked indicia be visibleeither as a light or a dark mark against the masked background in thelight.

In another embodiment, the plastic article containing the phosphorescentphosphor pigment is masked by coating with a masking paint or ink thatdecomposes, vaporizes or becomes colorless in the presence of laserenergy. When an indicia is laser marked on the article, the coating isdestroyed and the underlying phosphorescence is revealed to form aphosphorescent indicia. Such a laser marking would also be visible inthe light as a light mark resulting from the revealing of the underlyingplastic composition.

It may be desirable to produce laser marks with the highest visualcontrast between the mark and the color of the surrounding plastic.However, it is known that by varying laser parameters, such as amperage,pulse frequency and pulse duration in the case of Nd:YAG lasers, andsuch as energy density, peak power and pulse energy in the case ofTEA—CO₂ lasers, for example, laser marks of varying shades andintensities may be obtained. For example, Nd:YAG lasers may have anamperage range of about 7 to about 20 amps, a pulse frequency range of 1to about 20 kilohertz (KHz), and a scanning velocity of about 150 toabout 600 millimeters per second. A method for obtaining both light anddark laser marks of varying shades and intensities on a single plasticarticle by selectively controlling laser energy parameters, is disclosedin our co-pending and co-assigned U.S. patent application, Ser. No.08/874,401, entitled “Controlled Color Laser Marking of Plastics”, filedJun. 13, 1997, the disclosure of which is hereby incorporated byreference. The method may be used to obtain similar laser marks onphosphorescent plastic articles comprising the polymer compositions ofthe present invention to obtain laser designs that appear asmulti-colored designs in the light and that are visible in contrastagainst the phosphorescent plastic background in darkened environments.

To obtain both light and dark laser marks on a phosphorescent plasticarticle, the resin employed is a thermoplastic resin that foams whenheated by laser energy. Light marks are produced at certain selectedlaser settings by foaming of the resin and dark marks are produced atother selected laser settings by charring of at least the resin. Theremay also be a certain degree of charring of additives present in thecomposition. In this embodiment, the method for obtaining both light anddark laser marks on a phosphorescent plastic article comprises the stepsof providing a plastic article suitable for laser marking and having acomposition comprising (i) about 50% to about 99% by weight of athermoplastic resin that foams when heated, (ii) about 1% to about 50%by weight of a phosphorescent phosphor pigment, and (iii) about 0.001%to about 1% by weight of a laser energy absorbing additive and,preferably, about 0.01% to about 10% by weight of a lubricating agent;providing a laser having a principal wave length, a preselectable beamaperture and scanning velocity, and at least two variable laserparameters selectable from the group consisting essentially of anamperage from a range of amperages, a pulse frequency from a range ofpulse frequencies, a pulse duration rom a range of pulse durations, anenergy density from a range of energy densities, a peak power from arange of peak powers, and a pulse energy from a range of pulse energies,wherein each individual selection of two or more parameters comprises alaser setting that provides a laser beam having an individual laserenergy characteristic; selecting a first laser setting comprising aselection of at least two parameters to provide a first laser beamhaving an energy characteristic which causes the resin to foam;irradiating a first portion of the phosphorescent plastic article withthe first laser beam to produce a laser mark having a light shade;selecting a second laser setting different than the first laser settingand comprising a selection of at least two parameters to provide toprovide a second laser beam having an energy characteristic which causesat least the resin to carbonize; and irradiating a second portion of theplastic article with the second laser beam to produce a laser markhaving a darker shade than the light shade.

The method may further comprise the steps of (a) selecting a third lasersetting different than the first and second laser settings andcomprising a third selection of at least two parameters to provide athird laser beam having an energy characteristic which causes the resinto foam or at least the resin to carbonize; (b) irradiating a thirdportion of the phosphorescent plastic article with the third laser beamto produce another laser mark having a different light shade or adifferent dark shade on the plastic article; and (c) repeating steps (a)and (b) a desired number of times to produce a plurality of differentshades of light and dark laser marks on the phosphorescent plasticarticle.

The method may also produce light and dark laser marks on aphosphorescent plastic article that comprises a resin that is itselflaser markable without the addition of a laser energy absorbingadditive.

Examples of molded phosphorescent plastic articles having both light anddark markings of various shades and intensities according to selectedNd:YAG laser parameters of amperage and pulse frequencies areillustrated in FIGS. 1A, 1B, 2A, 2B, and 3. The exemplary phosphorescentpolycarbonate (PC) (FIG. 1), high impact polystyrene (HIPS) (FIG. 2) andhigh density polyethylene (HDPE) plastic articles were made from thecompositions illustrated in Table 1. The examples illustrate that lightlaser marks can be produced at low amperage and low pulse frequenciesand dark laser marks can be obtained at higher amperages and pulsefrequencies. Polycarbonate is a thermoplastic resin that foams whenheated and the foaming effect is enhanced by the use of a laser energyabsorbing additive, such as LS820. FIG. 1A illustrates laser marking ofa phosphorescent PC article without LS820, and FIG. 1B the laser markingof a phosphorescent PC article containing LS820. Similarly, FIG. 2Aillustrates laser marking of a phosphorescent HIPS article without (w/o)LS820 and FIG. 2B the laser marking of a phosphorescent HIPS articlecontaining LS820. FIG. 3 illustrates laser marking of a phosphorescentHDPE article containing LS 820. HDPE, which is transparent to laserenergy, requires a laser energy absorbing additive for laser marking.

Laser marked phosphorescent HDPE articles containing a whitening agent,0.03% zinc sulfide or 0.01% titanium dioxide respectively, areillustrated in FIGS. 4 and 5, respectively. The presence of a whiteningagent such as zinc sulfide or titanium dioxide in the concentrationsemployed in the invention compositions does not diminish thephosphorescence of the articles.

While the invention has been described herein with reference to thepreferred embodiments, it is to be understood that it is not intended tolimit the invention to the specific forms disclosed, on the contrary, itis intended to cover all modifications and alternative forms fallingwithin the spirit and scope of the invention.

TABLE 1 Example No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Resin* K-resinK-resin GPS GPS HIPS HIPS PET SAN SAN PC PC HDPE ABS ABS % LS820** 00.15 0 0.15 0 0.15 0 0 0.3 0 0.3 0.5 0 0.3 % A280*** 0.5 0.5 0.5 0.5 0.50.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 % Luminova ® 5.0 5.0 5.0 5.0 5.0 5.05.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 Glow effect 3 4 3 4 5 5 2 3 3 3 4 3 5 5Mark quality 3 4 3 4 5 5 2 3 3 3 4 3 5 5 Contrast 4 4 4 4 5 5 3 3 4 3 44 4 3 *K-resin = impact modified styrene-butadiene copolymer GPS =general purpose styrene HIPS = high impact polystyrene PET =polyethylene terephthalate SAN = stryene acrylonitrile copolymer PC =polycarbonate HDPE = high density polyethylene ABS = acrylonitrilebutadiene styrene **Iriodin/Lazer Flair Additive (mica based) ***Advawaxlubricating agent

We claim:
 1. A molded, extruded or formed phosphorescent plasticarticle, comprising: a composition that comprises (i) about 50% to about99% by weight of a thermoplastic or a thermosetting resin that absorbslaser energy, (ii) about 1% to about 50% by weight of a phosphorescentphosphor pigment, and (iii) optionally zero to about 10% by weight of alubricating agent, zero to about 2% by weight of a whitening pigment andzero to about 20% by weight of an inert filler; and a laser markedindicia on said phosphorescent plastic article.
 2. The article of claim1, wherein the composition comprises about 5% to about 20% by weight ofthe phosphorescent phosphor pigment.
 3. The article of claim 1, whereinthe composition comprises about 5% to about 10% by weight of thephosphorescent phosphor pigment.
 4. The article of claim 1 wherein thecomposition comprises about 5% by weight of the phosphorescent phosphorpigment.
 5. The article of claim 1 wherein the resin is an addition orcondensation polymer selected from the group consisting of polyesters,polystyrene, high impact polystyrene, styrene-butadiene copolymers,impact modified styrene-butadiene copolymer, poly-α-methyl styrene,styrene acrylonitrile copolymers, acrylonitrile butadiene copolymers,polyisobutylene, polyvinyl chloride, polyvinylidene chloride, polyvinylacetals, polyacrylonitrile, polyacrylates, polymethacrylates,polymethylmethacrylates, polybutadiene, ethylene vinyl acetate,polyamides, polyimides, polyoxymethylene, polysulfones, polyphenylenesulfide, polyvinyl esters, melamines, vinyl esters, epoxies,polycarbonates, polyurethanes, polyether sulfones, polyacetals,phenolics, polyester carbonate, polyethers, polyethylene terephthalate,polybutylene terephthalate, polyarylates, polyarylene sulfides,polyether ketones, and copolymers, grafts, blends and mixtures thereof.6. The article of claim 1, wherein the lubricating agent is selectedfrom the group consisting of silicone waxes, fatty acids, metallicsalts, ionomer waxes, amide waxes, hydroxy stearates, olefinic waxes,metallic stearates, complex esters, polyethylene waxes, polypropylenewaxes, and mixtures thereof.
 7. The article of claim 1, wherein thewhitening pigment is selected from the group consisting of about 0.001%to about 0.1% by weight titanium dioxide, about 0.005% to about 0.03% byweight zinc sulfide, about how much 0.005% to about 0.03% by weight zincphosphate, about 0.005% to about 0.03% by weight lithopone, about 0.005%to about 0.03% by weight zinc oxide, about 0.005% to about 0.03% byweight barium sulfate, and mixtures thereof.
 8. The article of claim 1,wherein the inert filler is selected from the group consisting ofcalcium carbonate, talc, china clay, and mixtures thereof.
 9. Thearticle of claim 1, wherein the composition further comprises about0.001% to about 20% of a laser energy absorbing additive.
 10. Thearticle of claim 1, wherein the laser energy absorbing additive isselected from the group consisting of about 0.001% to about 1% by weightof a mica-based laser energy absorbing additive, about 0.001% to about0.1% by weight carbon black, about 0.001% to about 0.1% by weighttitanium dioxide, about 0.01% to about 1% by weight barium sulfate,about 5% to about 20% by weight talc, about 5% to about 20% china clay,and mixtures thereof.
 11. The article of claim 1, further comprising acolor pigment that masks phosphorescence of the phosphorescent phosphorpigment, wherein the color pigment decomposes, vaporizes or becomescolorless in the presence of laser energy, and wherein the ratio of thephosphorescent phosphor pigment to the color pigment is about 150:1 toabout 1:1.
 12. The article of claim 11, wherein the color pigment iscarbon black.
 13. The article of claim 1 further comprising a coating onthe article, wherein the coating is present in an amount sufficient tomask phosphorescence of the phosphorescent phosphor pigment, wherein thecoating comprises a pigment that decomposes, vaporizes or becomescolorless when exposed to laser energy, and wherein the laser markedindicia on the article is phosphorescent.
 14. A molded, extruded orformed phosphorescent plastic article, comprising: a composition thatcomprises (i) about 50% to about 99% by weight of a thermoplastic resinthat foams when heated, (ii) about 1% to about 50% by weight of aphosphorescent phosphor pigment, and (iii) optionally zero to about 10%by weight of a lubricating agent, zero to about 2% by weight of awhitening pigment and zero to about 20% by weight of an inert filler;and both light and dark indicia laser marked indicia on saidphosphorescent plastic article.